Enhanced oil recovery (EOR) refers to a variety of processes to increase the amount of oil that can be recovered from an oil reservoir. These processes increase the permeation of oil to a production well in the ground formation and fall into three general categories; Chemical Injection, Gas Injection and Thermal Recovery. These processes can be accomplished by injecting a substance into the ground formation such as water, a water and surfactant mixture, or a gas stream, such as Carbon Dioxide, Nitrogen, or by heating the ground oil by injecting Steam. Typically, these processes reduce the oil's viscosity in the reservoir and provide a driving force allowing the ground oil to more readily permeate to a production well for extraction. Enhanced oil recovery offers prospects for withdrawal of more of the oil present in the reservoir and is typically used after primary and secondary withdrawal methods have been exhausted. In general, enhanced oil recovery processes are used to make non-productive reservoirs more productive.
A variety of enhanced oil recovery techniques attempted or in use include processes such as surfactant flooding, polymer flooding, and caustic flooding; miscible displacement processes such as miscible solvent flooding, carbon dioxide flooding; inert gas flooding, and foam displacement such as foam displacement variations of steam or hot water flooding, and thermal processes such as steam stimulation or cyclic steam injection, steam or hot water flooding, or in situ combustion.
The type of enhanced oil recovery technique used is based upon characterization of the reservoir. Characterization leads to an increased knowledge of the reservoir, including the ground formation, type and amount of oil present, depth of the oil, and pressure. Knowledge of these factors is critical to select the type of enhanced oil recovery technique. Each enhanced oil recovery process has its advantages, limitations, and disadvantages. For example, steam stimulation or cyclic steam injection employs heat to reduce the viscosity of the oil in the formation being treated. However, the temperature achievable by the steam is limited by the pressure of the formation. For example, Heavy crude oil-bearing formations are generally at a formation depth within 2,000 feet of the ground surface, and more typically are at a formation depth of about 1,000 feet from the ground surface. The temperature of steam injected is about 280° C. at a depth of about 1,000 feet and about 235° C. at a depth of about 2,000 feet. A disadvantage of this process is the steam condenses into liquid water in the formation which is immiscible with the oil and can be a factor in a host of other problems during and after extraction.
In miscible solvent flooding, the injected solvent is miscible in the oil and reduces its viscosity allowing oil to permeate and does not have the disadvantages associated with the immiscible liquid water formed in the steam stimulation process. A disadvantage of this process is the flooding solvent is a liquid and, therefore, unlike steam, only contacts a much smaller portion of the formation. Furthermore, the miscible solvent flooding process is not a thermal process and does not introduce any significant amount of heat to reduce the viscosity of the oil present in the formation. Many of these techniques have been hampered by high cost of injection materials, production of heat, generation of injection pressure and in some cases result in significant volumes of waste products leading to expensive clean up costs.
In nearly all enhanced oil recovery processes, the materials forming the stream to be injected into the well are transported from offsite locations. Additionally, in the case of thermal recovery processes, it is necessary to heat the materials prior to injection at or near the injection well head. Such transporting and heating add costs to the enhanced oil recovery and these costs usually have a significant energy-related component. The cost of solvent flooding chemicals in many cases inhibits their use despite technical feasibility. To avoid these costs, it is desirable to have a source of chemicals, heat, and pressure at the oil field, or more preferably in close proximity to the injection well for use in producing the injection stream. Processes that generate heat include combustion, geothermal, solar, and others. The combustion processes require fuel and tend to cause greenhouse gas emission. The solar processes tend to be limited to certain areas and do not have a well established technology. The geothermal processes are restricted to very few locations.
In light of the great demand for oil around the world, there is increasing demand for improvements in enhanced oil recovery methods and apparatus.